In the proposed research the high resolution structure of cowpea mosaic virus (CpMV) will be studied using single crystal X-ray diffraction procedures. With four other plant virus structures available at high resolution CpMV would represent a unique addition to this group in terms of structural properties (a T=1 virus with two types of protein subunit in the capsid), biological properties (a strategy of replication similar to the animal picornaviruses using post-translational processing of polyproteins to generate viable subunits) and physical properties (a covirus system containing three components, including a capsid devoid of RNA). Crystals of cowpea mosaic virus currently in use diffract X-rays to 3.5 Angstroms resolution, but have an extraordinary hexagonal unit cell with axial lengths of a=b=450, c=1038 Angstroms. Data have been collected to 10 Angstroms resoultion employing a rotating anode generator, focusing mirrors and a slit system to produce a highly collinated beam. Using the oscillation method and modifications to an available processing program to remove overlap contributions from neighboring reflections, data have been processed to 15 Angstroms resolution and an interpretable rotation function obtained. the structure at 15Angstroms resoultion is being determined by the molecular replacement real space phase refinement procedure. The high resolution structure determination described in this proposal will utilize synchrotron radiation to allow complete resolution of the diffraction maxima in the 1038Angstrom lattice direction. The high flux X-ray source will also permit data collection to at least 3.5Angstrom resolution. A combination of the isomorphous replacement and molecular replacement procedures will be used to solve the structure at high resolution. We propose to determine the structure of the empty capsid and one of the nucleoprotein components in order to establish the role of the nucleic acid in the capsid structure. The tertiary structure determination of this virus may define a new category of spherical virus protein subunit structure. The presence of more than one subunit type in the capsid will provide new information regarding quaternary structural interactions in viruses and serve as a model for RNA animal virus structure.